There are many known methods of forming and shaping steel. One method is to utilize a process known as continuous casting. This process, wherein liquid steel is poured directly into semi-finished shapes such as slabs, blooms, blanks or billets, is continuing to expand in its applications because, among other things, it eliminates or reduces the need for certain steelmaking equipment, compared to traditional casting of steel into ingots and later processing to desired products.
In the prior art, the continuous casting process produced a slab of steel from 150 to 300 mm thick and having a width up to 3,000 mm. These slabs were cut into pieces, of varying lengths, dependant upon process particulars. To produce .a flat rolled steel strip from that material, the discrete slab was reheated, passed through one or more hot rolling roughing millstands, and then passed through one or more hot rolling millstands that further reduced the thickness to approximately 2.5 mm. If necessary, it was then passed through at least one, usually several, reducing/finishing cold rolling millstands to obtain further reduction in thickness.
As the strip of steel got thinner in the hot rolling portion of the prior art process, it was difficult to get it to enter a millstand for further reduction in thickness. The steel strip entered each of the millstands at low speed and then was accelerated. It was important to try to access the tail end of the strip as fast as possible because that portion was the coldest by the time it entered the hot rolling millstands.
The need for creating discrete slabs from the as-continuously cast slab was definite and unavoidable, because of the entry and exit speeds of the various dissimilar types of apparatus combined into prior art systems. The known hot rolling millstand technology was not capable of speed-matching the roughing and finish millstands to the continuous output speed of known continuous casting apparatus, thereby preventing fully continuous operation. The required high speeds of the hot rolling mill, necessary particularly to avoid fire-cracking of the rolls and minimize heat loss, simply could not be matched up with prior devices by those skilled in the steelmaking art.
One of the problems in the system barring further reduction was that the hot steel strip became extremely difficult to control if it moved too fast from one process station to the next. A further difficulty of the discrete hot slab processes lay in threading the roll gap between the millstand rolls, which operation needed to be carried out for .each discrete slab. It required the opening of all of the millstands and then sequentially closing each stand, from the tail end of the slab towards the head or front end of the slab, until all were closed. Because of the heat loss occurring throughout each discrete slab, continued acceleration of the stands to effect rolling at a higher than desired hot rolling steady state speed was required to effect reduction before heat loss reached the point of non-workability of the steel.
The heat loss from the discrete slab was a serious problem because the tail end cooled rapidly, and often was below optimum hot rolling temperatures before it reached the last several millstands. To minimize this problem, the hot rolling millstands had to have said ability to constantly accelerate or, stated colloquially, to "zoom." Roughly speaking, the discrete slab had to enter each millstand at a very low speed, then be accelerated as quickly as possible to a speed in excess of desired hot rolling speed. The rapid acceleration or "zoom" was practiced to attempt to access the tail end of the discrete strip through all of the hot rolling mills as rapidly as possible, to even out any temperature drop and avoid heat loss to a level where the metal would be unworkable. For each millstand to "zoom", electric motors of horsepower and speed well above that required if a fully continuous, steady state hot rolling process could have been practiced, proved necessary. The use of a coil box, upstream of the first millstand, to provide a heat-retaining environment minimizing tail end cooling and cutting back the .level of acceleration required by the millstands, was the best solution afforded by the prior art to the need for "zooming." The capital costs of the coil box, however, offset any savings in electric motor costs, and the operating costs for utilities, though somewhat less, were still in excess of desired or acceptable limits.
The threading technique also required skill in manipulating. The speed of each discrete strip down the line, particularly after several of the stands had been closed and were "zooming" and taking their designed reductions.
While the theoretical minimum for strip thickness could be less than 1.5 mm, the substantial shortcomings in the prior art made the achievable hot rolled thickness no less than, at best, 1.8 mm to 2.5 mm. For applications requiring thinner gauges, the steel, after completion of hot rolling, had to be annealed, pickled and then cold rolled to the final thickness, additional processes that were time and energy consuming, and required substantial capital expenditures.
A general description of the relationship of continuous casting devices and rolling mills appears in"Rolling Mills Shape Up ", Iron Age (August 1990), p. 16 [which publication and its disclosures are not prior art to this invention].
A number of configurations of continuous casting devices and rolling mills were experimented with, in an attempt to develop a fully continuous casting-to-finished thin flat hot rolled steel strip process. Among the various mill configurations looked to for roughing levels of reduction were the planetary mill type, so-called because the work rolls orbited around a support structure of some particular configuration.
A planetary mill known as a "Platzer planetary mill" was developed in the late fifties and early sixties. It is generally described in U.S. Pat. Nos. 2,975,663; 2,960,894; and 2,709,934. The Platzer planetary mill is a force-fed mill having drive rollers that can accept a steel slab having a thickness of 50 to 100 mm and reduce it in thickness with planetary organized rolls to approximately a thickness of from 20 mm to about 3 to 6 mm. It was never a commercially successful device, mainly due to the fact that continuous casting of 50 to 100 mm thick slab was not achievable.
The prior art techniques for feeding the Platzer planetary .mill also presented serious shortcomings. When the thick, discrete slabs which were available from known continuous casting techniques were used, the force-feeding into the Platzer planetary mill created a large feed tongue or leading edge of steel strip, both initially and as the mill was screwed (adjusted) down to the final desired reduction. It was necessary to discard this feed tongue, usually by torch-cutting it free from the front end of the strip and discarding it upwardly, downwardly or transversely from the process line. The amount of metal wasted from each slab with respect to rolled strip product, although recycled into the melt end of the process, was substantial, particularly when related utilities, capital and operating costs were factored in.
Suggested prior combinations of continuous casting devices with Platzer mills, to comprise a hot steel strip system, did not include continuous hot rolling mill technology as part of the combination. For example, the Krupp/Platzer planetary mill, when combined with a continuous casting device, provided a hot strip mill with single pass thickness reduction of up to 98%. Muenker et al., Krupp/Platzer Planetary Mill, "Evolution, Design and Operating Experience in Ferrous and Non-Ferrous Practice" (February 1969); Fink, et al., "Economic Application of the Krupp/Platzer Planetary Mill For the Production of Hot Rolled Strip," Iron and Steel Engineer, January 1971, p. 45; Krupp/Platzer Planetary Mill--A Hot Strip Mill With Thickness Reduction of up to 98% (1987). The mill disclosed comprised a conventional continuous casting process allegedly configured for thin slab casting, which fed the as-cast slabs through conventional straightening rolls into a tunnel-type holding furnace. The as-cast slabs exited the holding furnace and passed into/were fed to the rolling gap of a Platzer planetary mill. (Usually, primary descaling would precede the feed rollers, with secondary descaling preceding the passing into/feeding into the Platzer planetary mill.) The Platzer planetary mill would reduce, in a single pass, the feed slab from its starting, as-cast and straightened thickness, up to 98%, to finished thickness. The resulting high reduction rolled steel strip was discharged from the mill onto a roller table by a standard pinch roll stand, which maintained tension between the roll gap and the pinch rolls. Cutting and coiling with conventional down-coiler units completed the disclosed process.
As an alternative to this arrangement, the Platzer planetary mill would reduce the feed slab from its starting, as-cast and straightened thickness, up to 98%. Instead of being discharged from the Platzer mill through a standard pinch roll stand/tension roller combination, the alternative configuration would utilize one or two (2) four-high finish millstands, particularly millstands fitted with Krupp IGC roll gap control system, disclosed to improve flatness and achieve close tolerances. No additional sources of heat to the steel strip were provided when the one or two (2) four-high finish millstands configuration were supplied, such that any possible finish reduction could not have been substantial because retained heat was inadequate.
The Muenker et al. article described in greater detail a portion of a configuration of a Platzer planetary mill combined with one or two (2) finishing mills, but not teaching the use of such configuration in combination with an as-continuously cast endless slab; Muenker et al. disclosed such mills for use only with discrete slabs. Muenker et al. described this alternative configuration as useful in a large tonnage situation, where the Platzer planetary mill served as a roughing millstand. FIG. 15 and the accompanying text compared a conventional hot rolling mill, utilizing twelve (12) horizontal and six (6) vertical stands, with a Platzer planetary mill roughing stand/finishing train comprising six (6) horizontal and two (2) vertical stands, both giving production rates of 150 tons/hour (pages 8-10; FIG. 15). Munker et al. disclosed the output dimension from the Platzer planetary mill of rough strip having a thickness of 10 to 20 mm.
Fink et al. addressed the use of a Platzer planetary mill in combination with a continuous slab caster and various downstream rolling devices. In the combination of continuous slab caster and Platzer planetary mill discussed there, Fink et al. noted that the feed rolls, used to force the individual abutted or discrete continuously cast slabs into the Platzer mill (p. 48), would take a 20% reduction, with the mill then taking an 80 to 98% reduction in one pass, depending upon the final thickness required. FIG. 4VI illustrated a furnace-planetary mill combination, again with the Platzer planetary mill being operated as a roughing millstand upstream of a five (5) to seven (7) stand finishing train, consisting of an undefined number of vertical and horizontal finishing millstands.
Besides the Platzer planetary rolling mill, the only other such mill used on a commercial scale was the Sendzimir planetary mill. Sendzimir planetary mills were generally described in a number of United States patents, including U.S. Pat. Nos. 2,932,997; 2,978,933; 3,049,948; 3,076,360; 3,079,975; 3,147,648; 3,138,979; 3,210,981; 3,533,262; and 3,789,646.
The differences between the Platzer planetary mill and the Sendzimir planetary mill were and remain well-known to one of ordinary skill in the art. In practical applications, it was known that a minimum feed slab thickness for a Sendzimir mill of at least about 120 mm was required to produce acceptable rolled product. For a given width, this greatly exceeded the minimum thickness which Platzer planetary mill technology would require. It was also well known that the rolled strip exiting from a Sendzimir planetary mill was not flat, exhibiting a marked scalloping or rippling in the rolling direction which required additional finishing mills to flatten the strip. The inability of a Sendzimir planetary mill to provide flat strip, in comparison to Platzer technology, was a direct result of the difference in construction between these types of planetary mill. Sendzimir planetary mills include a rotating beam, while Platzer planetary mills use a stationary back-up beam. The flow of metal through the Sendzimir mill, because of the rotating beam, is such that the scalloped or rippled strip results. The stationary back-up beam of the Platzer planetary mill establishes a metal flow during rolling that does not distort the strip, such that only a very slight, long wave in the longitudinal casting/rolling direction may result on occasion.
The fixed versus rotating beam difference between Platzer and Sendzimir planetary mill technology presents another advantage to use of Platzer technology. Because of the stationary back-up beam, it is possible, through use of various inserts in the beam, to provide a transverse (across the casting/rolling direction) profile to the slab by the rolling process. By use of such selected inserts, a Platzer planetary mill can provide an optimal profile to the output slab for further downstream processing, without the need for additional millstands dedicated to profiling the output sheet after reduction in the planetary mill.
The Platzer planetary mill is also capable of adjustment to close down the roll gap, allowing for optimization of the initial entry thickness and increased running reduction after threading. In contrast, the initial entry of the steel in a Sendzimir planetary mill cannot be adjusted down; it is established by the mill size itself, and cannot be varied.
With respect to operating costs, and maintenance, the Sendzimir planetary mill was more costly to use, primarily because of the roll gap friction difference over a Platzer planetary mill. Because of the configuration of the Sendzimir planetary mill, there is considerable friction between the work rolls and the slab being rolled. This causes increased wear on the work rolls and increased power consumption and motor sizing requirements, in comparison to a Platzer planetary mill. In a Platzer planetary mill, there is little friction between the work rolls and the slab; the main friction encountered is that in the bearings in the intermediate rolls. The result is that work roll life is longer, and operating and capital costs lower, than that of a Sendzimir planetary mill.
Sendzimir, "Hot Strip Mills for Thin Slab Continuous Casting Systems," Iron and Steel Engineer, October 1986, p. 36, described a proposed Sendzimir planetary mill layout, and illustrated several continuous casting/planetary mill and thin slab caster (Hazelett)/planetary mill combinations (see FIGS. 8-9). The basic planetary hot strip mill layout illustrated by Sendzimir (FIG. 1) comprised an edger and descaler preceding the feed rolls used to feed the slab into the roll gap of the planetary mill. Downstream take-off from the Sendzimir planetary mill was effected by a planishing mill acting through a set of tensioning rolls. A runout table, pinch rolls and carousel coiler completed the disclosed set-up.
(A planishing mill, as that term is understood by one of ordinary skill in the art, would provide less than a 10% reduction to the feed strip. In usual usage, a "planishing" mill would function substantially as a flattening device, which would, as part of that process, take no more than a maximum 3-5% reduction.)
The Sendzimir planetary mill was stated to be capable of a reduction in thickness of 95% in one pass. The feed rolls were stated to "push the slab, taking a small reduction, through a guide into the planetary rolls, where the main reduction is accomplished . . . " (p. 36). One or two sets of two high feed rolls were disclosed (pp. 36-37; FIG. 2). Sendzimir taught that the planetary mill should "be operated continuously, with [discrete] slabs being fed one butting against another and with the continuous, high temperature, high heat input furnace located in tandem with the mill. Slab temperature can be kept constant within precise limits and close gauge control of the finished strip is easily obtained. In fact, commercial cold rolling tolerances can be obtained directly from the hot mill, end to end, without any long, heavy leading or trailing ends. With automatic gage control at the planishing stand, an even finer adjustment will be obtained" (p. 37). In this configuration, Sendzimir was clearly not disclosing a fully continuous process using as-continuously cast endless slab steel directly from a continuous caster, but instead was describing a system for use with discrete slabs.
Sendzimir also disclosed allegedly experimental tandem operation of continuous casting devices combined with planetary mills:
Experimental tandem operation of casters and planetary mills PA0 More than 20 years ago, attempts were already being made to continuously roll slabs with the objective of converting the entire heat of the furnace into hot coils (FIG. 8). Numerous metallurgical, handling, reheating and surface problems were encountered. Balancing the output of the caster proved difficult together with handling the slab on the runout table, entry into the furnace, and operation of the planetary mill and coiler. PA0 An initial mold size of 21/2.times.171/2 in. [50.times.435 mm] was tried in Germany. It was too small and the speed of casting too slow for successful hot rolling downstream. With a slab speed of 4 to 5 fpm [1.5 m/min], the slab edges were black when entering the rolling mill. However, when everything was working properly, 80-in. OD coils were produced. PA0 Next, a high-tonnage, proven continuous caster coupled with a planetary mill in the U.S. provided slabs which entered the mill at 16 to 18 fpm [5 m/min]. The heat balance was correct and 60-ton hot coils were produced on an experimental basis. PA0 In a third attempt, in Austria, the objective was to put the planetary mill back to back in tandem with the caster, eliminating the heating furnace but considering use of an equalization hood and possibly an edge reheater. This scheme would have required allowing the dummy bar head from the caster to go through the planetary mill and be cut off by a flying shear just ahead of the coiler. Experiments were conducted with a planetary roll bite made directly into the cast section, with the mill screwdown coming on blocks to achieve the desired gage. The experiments were successful; a tapered section after the dummy bar head proved that only a small amount of the metal would have to be scrapped. PA0 New attempts in the future will utilize past experience and, at the same time, permit working with thinner cast sections from newer types of casters. For example, a mill is under consideration for rolling continuously cast sections of 2.times.50 in. [50.times.1,250 mm] and 11/2.times.50 in. [37.times.1,250 mm], but with both systems able to roll cast sections as thick as 3-in. for special products. PA0 Planishing mill--Downstream from the planetary mill, it may be desirable to include one or more planishing mills, depending on factors such as if the product is simple or sophisticated, whether the hot strip will be used directly or will be cold rolled, if metallurgical cleanliness or low cost is dominant in steel production, and whether the steel is a special type as such as low alloy high strength, high alloy, silicon or stainless. In deciding to include planishing mills, the need for heavy reduction after the planetary mill must be balanced against added investment cost and hot strip quality. PA0 A 10% reduction in the planishing mill might be sufficient for many applications, e.g., galvanized steel. Reductions of 35 to 50% might be appropriate for hot strip to be used for building construction where light reflection will accentuate surface detail. PA0 Normally, a simple 2-h mill could achieve a 10 to 12% reduction and eliminate most of the scallops. Although 3-h mills give reductions of up to 20%, work roll wear would make this solution questionable for mills operating continuously for 20-hr. periods. This could also apply to mills such as the 4 and 6-h type used at the Nippon Yakin 68-in. wide installation. Although these two types of mill could achieve reductions of 30 to 35% and provide good shape (especially the 6-h), work roll wear and the need for exchanging rolls would limit their application for long continuous runs. PA0 After the planishing mill, there should be a flying shear and a coiler. The coiler can be of the carousel type or two separate coilers can be used to handle the uninterrupted flow of strip. PA0 When the strip is parted by the shear, the trailing end must be accelerated away from the succeeding coil. A gap of 10 to 15 ft [3-4.5 m] is desirable so that the front end can be caught in the coiler without creating a stoppage. PA0 [I]n the production of thin steel strip, conventionally the starting material is thick steel slab, having a thickness of between 150 and 300 mm, which after being heated and homogenized at a temperature between 1,000.degree. C. and 1,250.degree. C. is roughened down to form an intermediate slab with a thickness of approximately 35 mm, which is then reduced to a thickness of between 2.5 and 4 mm in a hot strip finishing train consisting of several millstands. Further reduction to strip with a thickness of between 0.75 and 2 mm then takes place in a cold rolling installation. The previously pickled strip is cold reduced in a number of interlinked millstands, with addition of a cooling lubricant. Methods have also been suggested in which thin slabs are cast, and after being heated and homogenized, are passed direct to a hot strip finishing train. PA0 All such known and proposed rolling processes have been developed for discontinuous rolling operations. The casting of the slabs, the hot rolling of the slabs and the cold rolling of strip take place in different installations, which are effectively used only during a part of the available machine time. In a discontinuous rolling operation, it is necessary for the running of the installations to take into account the entry and exit of each slab and the temperature differences which can occur between the head and tail of each slab. This can lead to complicated and expensive measures. PA0 good results can be obtained when, after hot rolling of continuously cast steel slab in the austenitic region to form sheet, a further rolling of the thin sheet (2-5 mm) can take place at lower speeds (i.e., less than 1,000 m/min. preferably less than 750 m/min.), provided that this rolling is in the ferritic region, i.e., below temperature T.sub.1 (see below). This rolling is preferably followed by overaging at 300-450.degree. C. The result is a formable thin sheet strip which has good mechanical and surface properties and does not require cold-rolling. PA0 (a) in a continuous casting machine, forming liquid steel into a hot slab having a thickness of less than 100 mm. PA0 (b) hot rolling the hot slab from step (a), in the austenitic region and below 1,100.degree. C., to form strip having a thickness of between 2 and 5 mm. PA0 (c) cooling the strip from step (b) to a temperature between 300.degree. C. and the temperature T.sub.1 at which 75% of the steel is converted to ferrite. PA0 (d) rolling the cooled strip from step (c) at said temperature between 300.degree. C. and T.sub.1 with a thickness reduction of at least 25%, preferably at least 30%, at a rolling speed not more than 1,000 m/min., and PA0 (e) coiling the rolled strip from step (d). The temperature T.sub.1 in .degree. C. at which on cooling 75% of the austenite is converted into ferrite has a known relationship with the percentage of carbon in the steel, namely T.sub.1 =910-890(%C). PA0 The main reduction by the planetary millstand can lead to a very fine grain size which is undesirable for deep-drawing qualities. The second-stage small reduction of not more than 40% at the prevailing rolling temperature can then lead to a critical grain growth which converts the fine grains into more desirable coarse grains. A planetary millstand can give rise to the formation of a light wavy pattern in the sheet. By the further reduction in the planishing millstand it has appeared possible to remove this wave shape entirely. Optimum rolling conditions can be achieved in the planetary millstand if before hot rolling the slab is first passed through a homogenizing furnace and held at a temperature of 850-1,000.degree. C. preferably about 950.degree. C.
Page 39. FIG. 8, which included a slab cutting station between the continuous caster and the equalizing furnace, began the disclosed feeding sequence to the Sendzimir planetary mill, such that there again was no as-continuously cast endless slab of steel in the continuous casting/planetary mill combination. Plainly, Sendzimir's teachings in regard to those configurations were all directed to discrete, non-continuous slab rolling operations, even where the primary source of those discrete slabs was a continuous casting device.
Sendzimir also disclosed a thick-slab Hazelett caster/planetary mill combination (pp. 40-41, FIG. 9). The Hazelett caster "is used to produce 2-in. [50 mm] thick slabs which pass through a reheat furnace before entering a planetary mill followed by a planishing mill. Strip exits the planetary mill at a nominal thickness of 0.150 in. [3.8 mm] and from the planishing mill at a nominal thickness of 0.135 in. [3.4 mm]. The slab exists the Hazelett caster at 24.5 fpm [7.3 m/min] with the strip exiting the planetary mill at 327 fpm [98 m/min] and the planishing mill at 364 fpm [109 m/min]" (pg. 40).
Sendzimir addressed the particulars of the optional downstream planishing mill, with regard to both number and function:
Pages 41-42. The work roll wear problem in the three-high, four-high and six-high mills used in the noted combination was plainly quite serious. Any system which would adopt a casting campaign which would approach 20 to 24 hours in duration, or longer, would plainly exceed the disclosed operable periods in Sendzimir.
Discontinuous rolling with a reversing mill was disclosed by Sendzimir to solve this problem with thin-section casting systems. For such a system to function, Sendzimir indicated, the reversing mill would require elaborate, expensive electrical equipment of substantial speed and power. If continuous operation of the discontinuous rolling mill was sought, two hot coil boxes and their attendant substantial capital outlay would be required. The reversing millstand, in that case, could be a four-high or six-high mill, or a two-high mill, which "would permit heavier reduction in each finishing pass, thinner gages (e.g., 0.040 in.) [1.016 mm], and better gage accuracy."
Proposed Sendzimir planetary mill installations were purported to have used one or two (2) planishing mills, comprising three- and four-high millstands, effecting 14, 20% reduction (one planishing mill), or 26% reduction (first mill), and 23% reduction (second mill), when two (2) three-high millstands were used. Upstream feed roll reductions of 16, 20% (one feed roll) or 22% (first feed roll), 28% (second feed roll) were stated to also have been used, with two (2) feed rolls/two (2) planishing millstands in combination having been one configuration purportedly structured.
None of the prior art teachings concerning Platzer and/or Sendzimir planetary mills disclosed a fully continuous process wherein as-continuously cast endless slab was continuously converted to continuous steel strip, of such gauge/thickness and physical proporties to allow direct use in product manufacture without further processing, particularly cold rolling, without any discrete slab use. In each case, the configurations disclosed did not constitute a fully continuous operations, and did not provide adequate post-planetary mill reductions by hot rolling to achieve necessary thickness and physical properties in the product steel strip.
Despite the teachings of Muenker et al., Fink et al. and Sendzimir, and, in fact, in part because of them, then, the prior art was in actuality still left seeking a fully continuous system and apparatus to make hot rolled steel strip, which would function on the commercial scale under actual manufacturing conditions of strip width and thickness, needed operating efficiency and quality, and available capital and operating (including utilities) cost. None of these disclosures put one of ordinary skill in the steelmaking art into possession of a continuous system, capable of steady state operation at economic production rates, which processed as-continuously cast steel slabs into thin steel strip in one endless process.
Contrary to the implications or statements in the Muenker et al., Fink et al. and Sendzimir papers, discrete slabs could not simply be butted up against each other and force-fed into a planetary mill. Right-angled abutting front end (following slab) to tail end (leading slab) arrangements of successive discrete slabs would not consistently feed into a planetary mill. Slabs could bind and ride up, front end on leading tail end, or be accordioned by the entry. Damage to the mill would result, or loss of slabs. The front and tail edges of slabs would be shaped, such as by machining of cooled slabs, to make an operable process, which slabs would dovetail or mate to mimic an as-continuously cast slab. A chevron configuration was preferred, the tail end of the leading slab bearing a female shape resembling the tail end of an arrow, and the front end of the trailing slab bearing a male shape resembling an arrow head. This added substantial cost to the process, and increased processing time to a commercially unacceptable level.
Use of a series of discrete slabs in the prior art discontinuous sytems caused additional problems downstream of the rolling mills. Runout roller tables comprise roller and apron means over which the hot strip must be transported towards the down-coiler and its associated pinch roller. When the front end of the discrete strip begins its travel over the table, the strip thickness, strip speed and the friction encountered by the strip tends to intermittently bind and release it, causing buckling, deflection, distortion, and, in the worst case, causing the strip to fly away from the table. This causes damage to the strip or, in the case of table cobble, complete loss. Thus, transporting each strip down the table into the pinch roll and down-coiler risks these problems. With the discrete slab processes, this transporting and feeding through pinch rollers must be repeated with every new discrete strip, resulting in repeated risk of lost, defective strip and unacceptable process downtime.
Combinations of continuous casting devices with planetary mills, hot rolling mills and cold rolling mills were known. Hartog et al., EP 0 306 076, Method and Apparatus For The Manufacture of Formable Steel Strip, assigned to Hoogovens Group B.V. (published Mar. 8, 1989), disclosed several such combinations, to produce a formable steel strip with a thickness of between 0.5 and 1.5 mm (page 2, col. 1 11. 1-3). Hartog et al. was directed to a very specialized application, requiring the production of a very high quality ferritic steel, whose use for deep drawing applications was dependent on those special metallurgical properties.
Hartog et al. described the conventional method of production of steel strip, which their invention allegedly sought to improve upon:
Page 2, col. 1, 11. 10-38.
The supposed key to the Hartog et al. invention was the alleged discovery that
Page 2, col. 2, 11 35-46.
To produce the thin steel strip, Hartog et al. disclosed the sequential performance, in a continuous process, of the steps of:
Page 3, col. 3, 11. 5-23.
Hartog et.al. emphasized that their process allowed the casting of thin slabs, on the order of approximately 50 mm, instead of the known 150-300 mm slabs, with resulting savings in continuous casting device construction. The separation of the rolling in the austenitic region (step b) from rolling in the ferritic region (step d) by the step c cooling step, thereby avoiding so-called two-phase rolling, was critical to achieving good mechanical and surface properties independently of the speed of deformation, allowing lower speed operation than that disclosed as necessary by certain other art (page 2, col. 3, 11. 24-52). Up to 120 tons of steel, Hartog et al. disclosed, could purportedly be continuously cast into 0.5-1.5 mm sheet by their process, with virtual 100% use of continuous casting device material output, an allegedly superior result over prior art discontinuous processes starting from steel slabs having a maximum weight of 25 tons (page 2, col. 3, 1. 53-col. 4, 1. 10).
The ferritic cold rolling (400-600.degree. C.) portion of the Hartog et al. process required at least a 25% thickness reduction (page 2, col. 4, 11. 46-48). The austenitic hot rolling step preferably effected a considerable reduction in thickness in a few stages, including the planetary mill. Hartog et al. taught a "main reduction" in a planetary mill, after which a rolling reduction of not more than 40%, e.g., 10% to 20%, was applied in a "planishing" millstand, "in order to correct the shape of the strip and improve the crystal structure" (page 4, col. 5, 11. 34-43). The relationship between the planetary mill, the "planishing" mill, product flatness and grain size was set out:
page 11, col. 5, 11. 43-58.
FIGS. 1-3 disclosed several configurations of the Hartog et al. apparatus, each of which include a continuous caster followed by a homogenizing furnace, followed by a planetary mill, followed by a "planishing" millstand for hot rolling, followed by cooling means, and then followed by one or two (2) cold rolling, four-high millstands.
As for casting speed and reductions, Hartog et al. suggested that a continuous slab of about 50 mm thickness and width of about 1,250 mm be cast at a speed of about 5 m/min. with the planetary mill reducing same in one pass to a thickness of between 2 and 5 mm. The resulting very fine grained austenitic material, when next passed through the single hot "planishing" mill, underwent a maximum 40% further hot reduction. More particularly, Hartog et al. thought that, where a final steel strip thickness of between 0.6 and 1.5 mm was desired, the thickness before and after the cold mill (one or two (2) four-high millstands), needed to be adjusted to achieve a reduction of at least 25%, though "a reduction of more than 40%, e.g. 60%, should be sought" (page 5, col. 7, 11. 10-30; col. 7, 1. 57-col. 8, 1. 9). Use of two (2) four-high cold millstands was suggested where a certain ferritic reduction was desired for product quality, mostly where a high quality, deep drawing steel grade was desired, and a recrystallization annealing step, with necessary longer annealing time (10-90 seconds) furnace residence would necessarily follow the cold rolling (page 6, col. 9, 11. 13-27).
Hartog et al. plainly added nothing to the disclosures cf processing configurations incorporating Platzer and Sendzimir planetary mills, except the mandated use of a cold rolling operation as a critical part of the sequence.
The prior art thus failed to disclose a configuration or process which would result in the production of directly-usable, properly gauged, metallurgically acceptable strip steel, by a fully continuous process which did not use discrete slabs of cast steel, and failed to disclose a fully continuous process which could provide steel strip of thickness of less than 1.8 mm, without the need for cold rolling, from as-continuously cast endless steel slab.
The steelmaking art therefore had to cold roll and otherwise further process hot rolled strip steel before end product manufacturing thicknesses of less than 1.8 mm could be achieved, and the desired physical properties obtained. Capital outlay and operating expenditures remained substantial because of this need for cold rolling, as well as the failure to engage in fully continuous processing of the as-continuously cast endless steel slab.